I use corrugated cardboard cut from boxes (so far ones just in my house - diaper boxes, storage boxes etc. lol) but I cut it HALF the length of the strip I need (or the length I want the skirt to be plus 1 or 2" for the knot). Then you only cut at ONE end and you end up with strips two times the length of your cardboard. This way I don't need crazy long strips of cardboard!!
When you cut, you need to make sure your scizzors are in between the layers of cardboard, making it a straight cut!
HTH!

This makes a lot of since and I was going to suggest this as well. When I wrap my tulle around the cardboard piece, I try to keep it as even as possible and make sure it overlaps. When I cut I make long cuts instead of short ones. I've noticed that with the short cuts it tends to be more choppy, but the longer cuts give you more of a straight finish. I also hold my tulle down with four rubber bands (two at both ends).